Conditional lethal (ts) vaccinia mutants will be isolated and used in attempts to reactivate in vitro Molluscum contagiosum. Cell membrane modifications influenced by this virus will be compared with those produced by Yaba or rabbit fibroma, associated with benign tumors and in vitro transformation. Control by a single genetic determinant of the glycosylation of an induced plasma membrane polypeptide upon cell-cell interactions and spread of infection will be examined in the case of syncytiogenic or hemagglutinating variants of vaccinia. Induction of similar glycopeptides by Yaba, fibroma and Molluscum will be sought. De novo formation of poxvirus envelopes offers a unique model for studying membrane assembly. The role of this envelope in organization of sub-viral structure connected with maturation may be elucidated by means of suitable inhibitors and ts mutants with morphogenetic defects. Other mutants defective in early functions, particularly those related to the activity, conformation and packaging of DNA could be useful. Studies on association of a histone-like protein with the DNA may also provide insight into the cyclical folding and relaxation of the chromosome during virus development.